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Sun over Earth’s equator at equinox

Image at top by Tau’olunga via Wikimedia Commons.

The 2017 autumnal equinox for the Northern Hemisphere (spring equinox for the Southern Hemisphere) will take place on Friday, September 22, at 20:02 UTC. At this special moment – the instant of the September equinox – the sun is at zenith, or straight overhead, as seen from Earth’s equator. That’s the meaning of equinox. The September equinox sun crosses the sky’s equator, going from north to south.

Who will see the sun overhead at the moment of this year’s equinox? If you were on the sun on September 22 at 20:02 UTC, you’d see the hemisphere of Earth shown in the simulated image below. Looks like you’d have to be on a ship in the Pacific Ocean, some 38o south of San Francisco, California, to see the sun directly exactly directly overhead at noon at the exact moment of the equinox.

But no matter. Everyone along Earth’s equator on the day of the equinox – and for a day or two before and after it – will experience that noonday sun more or less overhead.

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Simulated image of Earth (without clouds) as “seen” from the sun at the instant of the September equinox (2017 September 22 at 20:02 Universal Time). Image credit: Earth and Moon Viewer

Although the equinox happens at the same instant for everyone worldwide, the clock time for the equinox varies by time zone. In the U.S., the local clock time for the September 22 equinox will be 4:02 p.m. EDT, 3:02 p.m. CDT, 2:02 p.m. MDT and 1:02 p.m. PDT.

On the day of the equinox, the sun rises due east and sets due west all over the world, with everyone worldwide receiving approximately equal portions of day and night.

When October comes rolling around that’ll change dramatically. By then, the sun will rise noticeably south of due east and will set noticeably south of due west. That’ll mean shorter days and longer nights for the Northern Hemisphere, and longer days and shorter nights in the Southern Hemisphere.

After the equinox, the sun (and migrating birds) will continue to travel southward to the southern climes. Arctic sea ice will begin to freeze; Antarctic ice will start melting. The great wheel of the seasons will continue to turn.

How to celebrate? Try to watch as the sun rises due east and sets due west on the day of the equinox. If you do that from your backyard, or deck, or a local park – somewhere that you have familiar landmarks – you’ll gain a handy tool for astronomy: that is, the tool of knowing the direction due east.

Day and night sides of Earth at the instant of the September equinox (2017 September 22 at 20:02 Universal Time). The shadow line at left (on the Atlantic Ocean) depicts sunset and the shadow line at right represents sunrise.

Bottom line: Around the equinox, the sun is overhead at noon for people at Earth’s equator.

The lunar calendars are almost here! View the moon phases throughout the year.

Everything you need to know about the September equinox 2016

Bruce McClure